Circuit for Power Amplification of an Input Signal and Signal Emission System Incorporating Such a Circuit

ABSTRACT

A circuit for power amplification of an input signal includes an input stage and an output stage, the said input stage including: a drive means incorporating a so-called main drive transistor, and a first so-called main input transistor able to receive the input signal, and mounted as a current mirror with the main drive transistor. The first main input transistor is coupled to the output stage via a second so-called main input transistor incorporated into the input stage and controlled by the drive means, the first and second main input transistors being coupled together and with the earth according to a structure of Darlington type by way of a resonant circuit.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to foreign French patent applicationNo. FR 0905399, filed on Nov. 10, 2009, the disclosure of which isincorporated by reference in its entirety.

FIELD OF THE INVENTION

The invention pertains to a power amplification circuit, in particularembodied on silicon and able to process signals whose frequency lies inthe X band (conventionally between 8 and 12 GHz).

BACKGROUND OF THE INVENTION

These circuits may for example be used in active transmit/receivemodules, such as those integrated within radars. More precisely, thesemodules comprise microchips able to phase shift and attenuate or amplifythe signal received or to be transmitted. The amplification function iscarried out by the amplification circuit which ideally must be capableof providing the necessary power required while guaranteeing goodlinearity of the transmit/receive chain.

An ideal amplification circuit must notably:

-   -   provide the power level required at output,    -   disturb the other circuits coupled upstream as little as        possible,    -   be sufficiently linear for the intended applications, and    -   allow control of the output gain,

all this with the lowest possible power consumption.

The amplification circuit is notably defined by its input impedance, itsgain and its transition frequency.

So that the operation of the circuit is optimal, it is preferable for itto function in its linear operating zone, so as to benefit from maximumand controlled values of the amplification gain.

This linear operating zone is made up of the frequencies lying between 0and the transition frequency Ft. The latter is defined by:

Ft=Fc*G max,

where:

-   -   Ft is the transition frequency,    -   Fc the cutoff frequency, and    -   Gmax, the maximum value that can be taken by the gain, in terms        of current, of the amplification circuit.

To be able to deliver maximum output, it is necessary to function atfrequency values for which the gain takes its maximum value, that is tosay in the linear operating zone of the circuit.

Hitherto, in amplification circuits, working at high gain values impliesa low input impedance, notably related to the use of a transistorself-biased by a current source, (for example a current mirror) andmounted in common emitter mode.

In this configuration (for transistors of judiciously chosendimensions), the input impedance is expressed in the form:

$\begin{matrix}{{{Ze} = \frac{\beta}{2\; {gm}}},} & (1)\end{matrix}$

where:

-   -   Ze is the input impedance of the amplification circuit, and    -   β is the gain of the self-biased transistor.

The disadvantage of a low input impedance is that:

-   -   it is harmful to the coupling with another circuit disposed        upstream, and    -   it decreases the value of the transition frequency, and        consequently reduces the extent of the linear operating zone of        the amplification circuit.

An aim of the invention is notably to solve these problems.

SUMMARY OF THE INVENTION

For this purpose, according to a first aspect of the invention, acircuit for power amplification of an input signal includes an inputstage and an output stage, the said input stage comprising:

a drive means incorporating a so-called main drive transistor, and

a first so-called main input transistor able to receive the inputsignal, and mounted as a current mirror with the main drive transistor.

According to a general characteristic of this aspect, the first maininput transistor is coupled to the output stage via a second so-calledmain input transistor incorporated into the input stage and controlledby the drive means, the first and second main input transistors beingcoupled together and with the earth according to a structure ofDarlington type by way of a resonant circuit.

Stated otherwise, while preserving the amplifying function of thecircuit, the structure of the self-biased transistor is notably replacedwith a structure of Darlington type.

The invention thus makes it possible to increase the value of the inputimpedance of the circuit, and consequently, that of the transitionfrequency. The linear operating span is thus increased.

Moreover, on account of the use of a resonant circuit for the couplingof the first and second main transistors, the amplification circuit isable to process signals whose frequency lies in the X band.

According to one embodiment, the first and the second main inputtransistor may be configured so as to be traversed by currents ofequivalent amplitude, to within an error.

For example, the said input signal may be of simple type, the saidsecond main transistor then being controlled by the main drivetransistor.

As a variant, the said input signal is of differential type, the inputstage then furthermore comprising:

a so-called auxiliary drive transistor incorporated within the saiddrive means,

a first so-called auxiliary input transistor, and

a second so-called auxiliary input transistor,

each so-called auxiliary element being mounted symmetrically with thecorresponding so-called main element.

According to one embodiment, the second main input transistor may becontrolled by the main drive transistor, and the second auxiliary inputtransistor may be controlled by the auxiliary drive transistor.

As a variant, the second main input transistor may be controlled by theauxiliary drive transistor, and the second auxiliary input transistormay be controlled by the main drive transistor.

Preferably, the first and the second auxiliary input transistor may beconfigured so as to be traversed by currents of equivalent amplitude, towithin an error.

According to one embodiment, the circuit may be embodied furthermore onsilicon.

According to another aspect of the invention, there is proposed a signalemission/transmission system, incorporating a circuit as mentionedhereinabove.

According to another aspect of the invention, there is proposed a use ofa signal emission/transmission system mentioned hereinabove, within aradar.

BRIEF DESCRIPTION OF THE DRAWING

Other advantages and characteristics of the invention will be apparenton examining the detailed description of a wholly non-limitingembodiment according to the invention and an appended single FIGUREwherein is represented an exemplary amplification circuit according tothe invention.

DETAILED DESCRIPTION

The single FIGURE is now referred to. The reference CIR denotes anamplification circuit.

The latter comprises an input stage EE coupled to an output stage ES.

The input stage is here formed of two symmetric branches BR and BRScoupled together via two coils BB1 and BB2, as is described in greaterdetail hereinafter.

The circuit CIR comprises two branches BR and BRS since an input signalof differential type is considered. In the case of an input signal ofsimple type, the circuit CIR would naturally be formed of just one ofthe two branches.

The branch BR will be described in detail. To each so-called mainelement of the branch BR there corresponds a so-called symmetricauxiliary element within the branch BRS. The references of theseauxiliary elements comprise a suffix “s”, as represented in the FIGURE.So as to simplify the description, only the branch BR will be described,the branch BRS being similar thereto.

A bias current Ip is delivered as input to the input stage EE, both onthe branch BR and on the branch BRS, given that an input signal ofdifferential type is considered.

The differential input signal is applied in the form of a differentialvoltage VeVes, respectively to the input terminals En and Ens of thebranches BR and BRS of the circuit CIR. More precisely, the differentialinput voltage is applied to the respective base of a first maintransistor T1 and of a first auxiliary transistor T1 s by way of twocapacitors referenced Cp and Cps.

The bias current Ip is delivered as input to a main bias transistor T3,more precisely on its collector.

The main bias transistor T3 is mounted as a current mirror with thefirst main transistor T1. Stated otherwise, the base of the main biastransistor T3 is linked to the base of the first main transistor T1,here by way of two resistors Ra and Rb mounted in series. Theirrespective value may be of the order of some hundred ohms.

So as to limit the losses of the drive current transmitted by the mainbias transistor T3, a transistor T4 is also connected between thecurrent source delivering the bias current Ip and the node common to thetwo resistors Ra and Rb.

This transistor provides an additional, so-called base, current whosevalue corresponds to the value of the current dissipated by the mainbias transistor T3.

The emitter of the main bias transistor T3 is linked by way of a coilBB1 to the emitter of the auxiliary bias transistor T3 s. The twotransistors T3 and T3 s form a drive means MP.

Likewise, the emitter of the first main input transistor T1 is linked byway of a coil BB2 to the emitter of the first auxiliary input transistorT1 s.

These coils BB1 and BB2 make it possible to decouple the transistors T3,T3 s, T1 and T1 s from the earth, by creating a high impedance at theworking frequency.

Moreover, the coil BB2 forms a resonant circuit of LC type with thecapacitor Cp, this resonant circuit coupling the transistors T1 and T2together and with the earth symbolized by a white arrow. This resonantcircuit enables the amplification circuit to process signals whosefrequency lies in the X band.

More precisely, the first main input transistor T1 is mounted accordingto a structure of Darlington type, well known to the person skilled inthe art, with a second main input transistor T2. Stated otherwise, thecollector of the second main input transistor T2 is coupled to thecollector of the first main input transistor T1. Moreover, the base ofthe second main input transistor T2 is coupled to the emitter of thefirst main input transistor T1 via a capacitor Cp.

The first and second main input transistors are configured in such a waythat they are traversed by currents of similar values, to within anerror.

More precisely, we have (for the branch BR):

${{Ze} = \frac{Ve}{Ie}},$

where

-   -   Ze is the input impedance of the branch BR of the amplification        circuit,    -   Ve the input voltage, and    -   Ie the input current.

Now:

Ve=Vbe1+Vbe2,

where

-   -   Vbe1 is the base/emitter voltage of the transistor T1, and    -   Vbe2 is the base/emitter voltage of the transistor T2.

For the sake of simplification, it is considered that Vbe1=Vbe2. Itfollows that:

Ve=2*Vbe2.

Consequently, we obtain:

$\begin{matrix}{{{Ze} = {\frac{2\; {Vbe}}{Ib} = {\frac{2\; {Vbe}}{\frac{Ic}{\beta}} = {{\frac{2\; {Vbe}}{gmVbe}*\beta} = \frac{2\; \beta}{gm}}}}},} & (2)\end{matrix}$

where:

-   -   Ib is the base current of the transistor T1 (or T2),    -   Ic is the collector current of the transistor T1 (or T2),    -   gm is the value of the transconductance of the transistor T1 (or        T2), and    -   β is the gain of the transistor T2 (or T1).

The input impedance according to the invention (equation (2)) is thusmuch greater than the input impedance of the circuits according to theprior art (see equation (1) hereinabove), improving the coupling with anoptional circuit coupled upstream.

Consequently, the current gain G of the branch BR of the circuit CIR canbe written:

${G = {\frac{Itot}{Ie} = {\frac{2*{gm}*\frac{Ve}{2}}{\frac{Ve}{Ze}} = {{Ze}*{gm}}}}},$

where:

-   -   G is the current gain of the circuit CIR,    -   Itot is the total current delivered as output from the branch BR        of the circuit CIR,    -   Ie is the input current of the branch BR of the circuit,    -   gm is the value of the transconductance of the transistor T1 (or        T2).

Thus, the increase in the value of the input impedance Ze implies anincrease in the value of the current gain G of the circuit.

As:

${{Ft} = \frac{jG}{2}},$

with Ft, the transition frequency, the increase in the current gain Gdoes indeed imply an increase in the value of the transition frequency.

The same calculation applies to the branch BRS of the circuit CIR.

Thus, the extent of the linear operating zone of the circuit CIR isgreatly increased. Consequently, it is possible to benefit from themaximum value of the gain of the circuit for a wider range offrequencies.

As illustrated in the FIGURE, the base of the second main inputtransistor T2 is linked to the base of the auxiliary bias transistor T3s.

Thus, the transistor is driven in dynamic mode by the first main inputtransistor T1. In static mode, the second main input transistor T2 isbiased by the main bias transistor T3 s.

As a variant, the base of the second main input transistor T2 may belinked to the base of the main bias transistor T3 and the base of thesecond auxiliary input transistor T2 s linked to the base of theauxiliary bias transistor T3 (as illustrated in the FIGURE). Thecrossover illustrated in the FIGURE allows the variations in one branchto be passed on to the other branch and thus to preserve a perfectlysymmetric differential signal.

Moreover, the second main input transistor T2 is cascode mounted with atransistor T5. The emitter of the second main input transistor T2 isearthed and its collector is coupled to the emitter of the transistorT5. The base of the emitter T5 is linked to a reference voltage terminalVref.

The transistor T5 forms a buffer between the input stage EE and theoutput stage ES by:

-   -   imposing a voltage on the collector of the second main input        transistor T2,    -   by limiting the impedance on the collector of the second main        input transistor T2, and    -   by limiting the circuit's Miller effect, that is to say, the        influence of the amplification circuit's own gain on input        characteristics, in particular, the reduction in the input        impedance.

The collector of the transistor T5 is coupled to the output stage ES.The latter comprises for this differential mode a balun BL, that is tosay an electrical circuit able to effect the link between parallelprinted lines and a line printed above an earth plane.

The balun BL is powered by a supply voltage Vcc, and the output voltagebetween an output terminal St of the circuit CIR and the earth isreferenced St.

The supply voltage Vcc is also used to power all the active elements ofthe circuit CIR. The connections are not represented for simplificationreasons.

Another advantage of the amplification circuit CIR according to theinvention is that it is particularly compact. Indeed, since thetransistor T3 (or T3 s depending on the configuration chosen)participates directly in the static biasing of the transistor T2 (or T2s), it is particularly advantageous to place it in proximity to thetransistors T2 (or T2 s). This results in a circuit of smallproportions.

The transistors used in this example are of bipolar on silicon substratetype. It is also possible to use bipolar transistors on substrate ofAsGa or InP type.

1. A circuit for power amplification of an input signal comprising aninput stage and an output stage, the said input stage comprising: adrive means incorporating a so-called main drive transistor, and a firstso-called main input transistor able to receive the input signal, andmounted as a current mirror with the main drive transistor, the firstmain input transistor being coupled to the output stage via a secondso-called main input transistor incorporated into the input stage andcontrolled by the drive means, the first and second main inputtransistors being coupled together and with the earth according to astructure of Darlington type by way of a resonant circuit.
 2. A circuitaccording to claim 1, wherein the first and the second main inputtransistor are configured so as to be traversed by currents ofequivalent amplitude, to within an error.
 3. A circuit according toclaim 1, wherein the said input signal is of simple type, the saidsecond main transistor being controlled by the main drive transistor. 4.A circuit according to claim 1, wherein the said input signal is ofdifferential type, and in which the input stage furthermore comprises: aso-called auxiliary drive transistor incorporated within the said drivemeans, a first so-called auxiliary input transistor, and a secondso-called auxiliary input transistor, each so-called auxiliary elementbeing mounted symmetrically with the corresponding so-called mainelement.
 5. A circuit according to claim 4, wherein the second maininput transistor is controlled by the main drive transistor, and thesecond auxiliary input transistor is controlled by the auxiliary drivetransistor.
 6. A circuit according to claim 4, wherein the second maininput transistor is controlled by the auxiliary drive transistor, andthe second auxiliary input transistor is controlled by the main drivetransistor.
 7. A circuit according to claim 4, wherein the first and thesecond auxiliary input transistor are configured so as to be traversedby currents of equivalent amplitude, to within an error.
 8. A circuitaccording to claim 1, embodied furthermore on silicon.
 9. A signalemission/transmission system, incorporating a circuit according toclaim
 1. 10. Use of a signal emission/transmission system according toclaim 9, within a radar.